2010 AMC 10B Problem 23
Attempt Problem 23 of the 2010 AMC 10B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AMC 10B solutions, or check the answer key.
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23.
The entries in a array include all the digits from through arranged so that the entries in every row and column are in increasing order. How many such arrays are there?
Answer: D
Solution:
Let be the entry in row and column The increasing conditions force and to be or
If then Choose which two of go in the bottom-left pair the other two go in the top-right pair Each pair then has only one increasing order. There are arrays: two orders for around the upper-left corner and choices for the bottom-left pair. By reversing the digits and rotating the array, there are also arrays with center
If choose the three entries in positions They can be any three of except or those two choices would put all three small or all three large entries on one side and violate a required comparison with the center. Every other choice uniquely determines the remaining entries and their increasing orders. This gives arrays.
The total is
Thus, D is the correct answer.
Problem 23 in Other Years
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